The present invention relates to the manufacture of optical waveguide fibers.
Optical waveguide fibers have been greatly improved during the last decade until attenuations less than 1.0 dB/km have been routinely achieved. Such low loss fibers are formed by a chemical vapor deposition technique (CVD) which results in the formation of extremely pure materials. In accordance with this technique, optical waveguide preforms can be formed by depositing glass layers on the outside surface of a temporary mandrel or on the inside surface of a tube which later forms at least a portion of the cladding material. These two embodiments of the CVD technique will be briefly described below.
In one embodiment of the CVD process the vapor of reactant compounds is introduced into a flame where it is oxidized to form a glass soot which is directed toward a mandrel. This so-called flame hydrolysis method of formng coatings of glass soot is described in greater detail in U.S. Pat. Nos. 3,737,292; 3,823,995; 3,884,550; 3,957,474 and 4,135,901. To form a step-index optical waveguide fiber, a second coating having a lower refractive index than the first is applied over the outside peripheral surface of the first coating. To form a gradient index fiber, a plurality of layers of glass soot are applied to the starting member, each layer having a progressively lower index of refraction as taught in U.S. Pat. No. 3,823,995. Gradient index fibers may also be provided with a coating of cladding material. After the plurality of coatings are formed on the mandrel, the mandrel is removed and the resultant tubular preform is gradually inserted into a consolidation furnace, the temperature of which is sufficiently high to fuse the particles of glass soot and thereby consolidate the soot preform into a dense glass body in which no particle boundaries exist. The resultant consolidated blank is drawn into an optical waveguide fiber.
In accordance with a second embodiment of the CVD technique the reactant vapor, together with an oxidizing medium, flow through a hollow, cylindrical substrate. The substrate and the contained vapor mixture are heated by a source that moves relative to the substrate in a longitudinal direction, whereby a moving hot zone is established within the substrate tube. A suspension of particulate material which is produced within the hot zone travels downstream where at least a portion thereof comes to rest on the inner surface of the substrate where it is fused to form a continuous glassy deposit. After suitable layers have been deposited to serve as the cladding and/or core material of the resultant optical waveguide fiber, the temperature of the glass tube is generally increased to cause the tube to collapse. The resultant draw blank is then drawn in accordance with well known techniques to form an optical waveguide fiber having the desired diameter. Such process parameters as temperatures, flow rates, reactants and the like are discussed in the publications: J. B. MacChesney et al., Proceedings of the IEEE, 1280 (1974) and W. G. French et al., Applied Optics, 15 (1976 ). Reference is also made to the text Vapor Deposition edited by C. F. Powell et al., John Wiley & Sons, inc. (1966).
Although the CVD technique of forming optical waveguide preforms results in the formations of optical waveguide fibers having extremely low attenuation, this method is also relatively expensive. For this reason research is being conducted on other fiber manufacturing techniques. The double-crucible process is widely recognized to be a low-cost process for making optical waveguides. While improvements have been made in both the composition of double-crucible fibers and apparatus for making such fibers as evidenced by U.S. Pat. Nos. 4,197,136 and 4,145,200, for example, such fibers are not expected to exhibit the very low-losses and high bandwidth which can be achieved by fibers formed by a CVD process.
A hybrid technique is disclosed in U.K. Patent application GB 2,023,127 A (equivalent to U.S. Pat. No. 4,249,925). A glass rod, preferably pure SiO.sub.2 or SiO.sub.2 doped with an oxide which modifies the refractive index, expansion coefficient or viscosity of the glass, is drawn into a fiber which is thereafter provided with one or more layers of cladding material. In one embodiment the core fiber, which has an expansion coefficient of 28.times.10.sup.-7 /.degree. C. is drawn through a crucible containing cladding glass having an expansion coefficient of 33.times.10.sup.-7 /.degree. C. This combination of core and cladding expansion coefficients results in a relatively weak fiber. In another embodiment a core fiber having an expansion coefficient of 32.times.10.sup.-7 /.degree. C. is coated by the flame hydrolysis process with a layer of germania silicate glass having an expansion coefficient of 25.times.10.sup.-7 /.degree. C., the resulting fiber being drawn through a crucible wherein it is provided with the outer cladding layer. The resultant fiber is again relatively weak. Moreover, the core-cladding interface is less than optimal, it having been formed by adding cladding glass layers to the initially formed core glass fiber.